The sidewalls of modern tires must meet performance standards which require a broad range of desirable properties. Thus, rubber compositions suitable for tire sidewalls should exhibit not only desirable strength and elongation, particularly at high temperatures, but also reduced heat buildup, low permanent set, excellent adhesion to adjacent portions of the cured tire. Additionally, good flex fatigue resistance and ozone and ultraviolet resistance are further requirements of all sidewall compositions.
It has been recognized that this complex balance of properties, required in the modern tire, cannot be satisfied by a single polymer and that, therefore, a blend of polymers must be employed. See, e.g., J. Walker et al, "Specialty Elastomers in Tires", Elastomerics, September 1985, pp. 22-29. It is noteworthy that all of the polymers employed in the blends described in such article possess a high degree of unsaturation. Consequently, such blends are readily vulcanized by the use of accelerated sulfur curative systems.
In the past, covulcanizates of EPDM and highly unsaturated rubbers have generally exhibited poor strength and high hysteresis when cured with conventional accelerated sulfur vulcanization systems. Accordingly, several different approaches have been undertaken to find a method of achieving a desirable covulcanization of blends of rubber of varying degrees of saturation, particularly blends of highly unsaturated rubbers and EPDM.
One frequently employed approach has been to halogenate the highly saturated rubber (typically "EPDM") thereby rendering a blend of such halogen-modified rubber with an unsaturated rubber sulfur curable. Thus, Schoen et al (U.S. Pat. No. 3,936,430) shows sulfur-curable blends of halogenated EPDM with unsaturated rubbers. (It is noteworthy that Schoen et al indicate at Column 4, lines 45-46 that the use of peroxide is "unnecessary".) Similarly, Hopper (U.S. Pat. No. 4,017,468) shows sulfur-curable blends of N-chlorothiocarboxylic amides or imides with unsaturated rubbers, whereas Kresge et al (U.S. Pat. No. 3,524,826) discloses sulfur-cured covulcanizates of brominated EPDM and highly unsaturated rubbers. In the same vein, Landi et al (U.S. Pat. No. 3,956,247) show a sulfur-curable blend of halogenated EPDM and diene rubber, and Hopper (in U.S. Pat. No. 3,915,907) shows sulfur curable blends of N-chlorothiosulfonamide-modified EPDM and diene rubbers.
Somewhat similarly, Son et al (in U.S. Pat. No. 3,897,405) show blends of (i) EPDM having --SH or --S--N linkages grafted thereto, with (ii) highly unsaturated rubbers, which blends are sulfur curable.
A second general approach taken to achieve the cocure of otherwise incompatible rubbers involves the use of a particular sulfur-cure accelerator or combination of sulfur-cure accelerators in conjunction with the use of sulfur. Thus, Mastromatteo et al (in U.S. Pat. No. 3,678,135) show the use of long chain hydrocarbon dithiocarbamate accelerators, such as zinc N-dodecyl,N-isopropyl dithiocarbamate in the sulfur cure of EPDM with highly unsaturated rubbers, while, similarly, Taylor et al (in U.S. Pat. No. 4,008,190) show the sulfur cure of EPDM with highly unsaturated diene rubbers employing a N,N-(higher alkyl)thiocarbamylsulfenamide as a vulcanization accelerator. In a similar manner, Hines et al (in U.S. Pat. No. 3,343,582) disclose a vulcanizable composition comprised of polyisoprene and EPDM, which composition contains as cure accelerators a benzothiazyl sulfide compound, an aryl-substituted guanidine and a thiuram sulfide.
Other approaches taken in the past to achieve such a cocure include that of Knabeschuh et al (in U.S. Pat. No. 3,817,952) which involves preparing a branched EPDM by heating EPDM with sulfur or a peroxide, and then blending such branched EPDM with a diene to form a sulfur or peroxide curable composition; that of Marchesini et al (U.S. Pat. No. 3,299,175) which employs the polymerization product of divinyl benzene and a vinyl unsaturated polymer as a filler for a peroxide-curable ethylene/alphaolefin copolymer (although Marchesini does state that sulfur can be employed in addition to peroxide, all Examples show peroxide only); and that of Wei et al (U.S. Pat. No. 3,179,718) which shows a vulcanized blend of nitrile rubber and ethylene/propylene copolymer which blend has been vulcanized employing both an organic peroxide and elemental sulfur. (It is noteworthy that Wei et al mentions neither EPDM nor sulfur cure accelerators.)
Both Bohm et al (U.S. Pat. No. 4,350,795) and Petersen (U.S. Pat. No. 4,202,801) show thermoplastic elastomers comprised of blends of a polyalphaolefin, EPDM and a conjugated diene rubber. While both these patents state that combination of sulfur and peroxide may be employed, Bohm et al show the use of sulfur only while Petersen exemplifies the use of peroxide only. Moreover, it is readily apparent that thermoplastic elastomers, due to their thermoplastic nature, are unsuitable for use as tire tread compositions.
U.S. Pat. No. 3,937,862 teaches the use in sidewalls of a mixed sulfur and peroxide vulcanization system (Example #23) with an EPDM polymer having a relatively low molecular weight (Mooney viscosity of 84 at 100.degree. C.) and concludes that it is not of any particular advantage compared to an all peroxide vulcanization system.
The applicants have surprisingly and unexpectedly found that when a high molecular weight (at least 6.0.times.10.sup.5 Mw) EPDM is utilized in a sidewall composition containing a three component cure system similar to the one in 3,937,862 that the ability of the sidewall composition to adhere to the adjacent carcass and tread compositions of the cured tire is dramatically improved. In addition, the internal heat buildup and Permanent set as measured in ASTM D623-78 method A are both dramatically improved over the results obtained using the lower molecular weight EPDM of 3,937,862.
Thus, it would be desirable to possess a tire having a sidewall composition which exhibits desirable strength and elongation as well as reduced heat buildup; low permanent set, excellent adhesion to adjacent portions of the cured tire.
Accordingly, it is an object of this invention to provide a tire having a sidewall composition which exhibits desirable strength and elongation as well as reduced heat buildup; low permanent set, excellent adhesion to adjacent portions of the cured tire.
It is a further object of this invention to provide a tire having a sidewall composition comprised of a covulcanizate of a highly unsaturated and high molecular weight saturated rubber.
These objects, and other additional objects will become more apparent from the following description and accompanying Examples.